In the aluminum industry and in the production of other metals and alloys, refractory products are frequently employed. In use, typically the refractories are in direct contact with a particular metal or alloy manufactured in its molten state.
Silica, in some form or the other, is often present in refractory articles and it is well known that aluminum, magnesium and lithium alloys with magnesium (especially in their molten state) are extremely aggressive in their attack of silica-based refractories. Solutions have been proposed in the past to this problem of rapid deterioration of refractory articles due to attack by molten metals and alloys. Some of these solutions will now be discussed in greater detail below, to highlight the differences between, and inadequacies of these techniques in comparison with the method of the present invention.
U.S. Pat. No. 4,681,81 9 ("the '819 patent") provides a method for treating a formed refractory article for use in contact with molten aluminum metal, to protect against attack by the metal on the surface of the article. The method of the '819 patent comprises impregnating the surface of the formed refractory article with an aqueous solution of magnesium fluosilicate for establishing thereupon a magnesium fluosilicate deposit capable of providing, upon thermal decomposition, fluoride values of a type and in an amount effective to enhance the resistance of the surface portion to attack by molten aluminum, and then heating at least the impregnated surface portion to decompose the magnesium fluosilicate to produce, in situ, the fluoride values. Magnesium fluosilicate is described as embracing MgSiF.sub.6.6H.sub.2 O as well as anhydrous MgSiF.sub.6.
The '819 patent further makes it clear that the fluoride values obtained by this process ordinarily or typically comprise the decomposition product MgF.sub.2. At column 6, lines 48 through column 7, line 2, the '819 patent describes in detail the protective fluoride values obtained due to use of MgSiF.sub.6 or its solution. In addition to MgF.sub.2, the '819 patent reveals that SiF.sub.4 can be another decomposition product which would make up the fluoride values. The fluoride values may also comprise calcium fluoride, namely CaF.sub.2. In case of situations wherein the article treated is a fibrous aluminum silicate, the fluoride values may also include SiO.sub.4 (F,OH).sub.2. Importantly, the '819 patent does not eliminate the formation of skulls on the formed refractory articles, upon exposure to molten aluminum or to other molten metals or alloys.
The present invention, instead of using complex and expensive materials such as MgSiF.sub.6 and other materials with similar properties, structure and elemental composition which would in fact produce skulls as produced by MgSiF.sub.6, and introduce unwanted materials such as boron or silica into the protective material formed on the refractory article, uses hydrofluoric acid solutions for the same purpose. The use of hydrofluoric acid makes the process of the present invention much less expensive than if one were to use MgSiF.sub.6 or its solution.
U.S. Pat. No. 5,033,721 is assigned to the owner of the '819 patent. U.S. Pat. No. 5,033,721 relates to insulating light-weight refractory materials. The principal method of U.S. Pat. No. 5,033,721 involves compositions including ceramic fibers, wollastonite powder, aqueous colloidal silica, unmilled vermiculite for forming refractodes. The compositions of U.S. Pat. No. 5,033,721 can also be improved for better resistance to chemical attack by treatment in accordance with the '819 patent with MgSiF.sub.6 or its solution. U.S. Pat. No. 5,033,721 and its use of MgSiF.sub.6 presents the same problems faced by refractory articles treated with MgSiF.sub.6 in the '819 patent.
U.S. Pat. No. 3,535, 154 ("the '154 patent"), is directed to the protection of substrates different from those which would be protected by the present invention. The '154 patent is directed to the protection of silicon nitride molded bodies, by dipping the bodies in an aqueous alkali metal, alkaline earth metal or heavy metal salt and then by calcining in an oxygen containing gas at a temperature of 1,200 to 1,500 degrees centigrade.
U.S. Pat. No. 4,492,721 ("the '721 patent"), provides a method of providing magnesium fluoride layers. These layers are provided for forming an antireflective layer and not for the purpose of forming an aluminum nonwettable refractory. The solutions utilized in the '721 patent are organic compounds and not inorganic compounds. The '721 patent is thus much closer to the '819 patent and to U.S. Pat. No. 5,033,721 than to the method of the present invention.
The present invention provides a method of making formed refractory articles non-wettable to molten aluminum, other molten metals and other molten alloys, by contacting the formed refractory article with a solution of hydrofluoric acid for a predetermined amount of time and drying at a temperature above 200.degree. centigrade to react the fluorine in the hydrogen fluoride with the refractory and drive off any free fluorine.